A coated type magnetic recording medium has been prepared by dispersing ferromagnetic particles in a binder, coating the resulting magnetic coating composition on a non-magnetic support, and conducting magnetic orientation, as well as a smoothing treatment, if necessary.
Various improvements have recently been made as better characteristics have been required for a magnetic recording medium (hereinafter referred to as a magnetic tape). One of the characteristics required to be improved is the S/N ratio (signal/noise). In order to improve the S/N ratio of a coated type magnetic tape, it is proposed to use ferromagnetic particles having a smaller particle size, that is, having a larger specific surface area. In this case, it is necessary that ferromagnetic particles should be homogeneously dispersed in a binder and that the coated surface of a magnetic layer should be smooth. However, as the particle size of ferromagnetic particles becomes smaller, it is more difficult to homogeneously disperse the ferromagnetic particles in the binder, whereby magnetic orientation of the ferromagnetic particles and surface smoothness of the magnetic layer deteriorate, and thus a higher S/N ratio cannot be obtained.
In order to improve dispersibility of ferromagnetic particles, various binders and dispersing aids have been studied, and mixing and kneading methods using various mixing and kneading devices (e.g., a two-roll mill, a three-roll mill, an open kneader, a pressure kneader or a continuous kneader) have been also studied as described, for example, in T. C. PATTON: Paint Flow and Pigment Dispersion (John Wiley & Sons, 1964), and U.S. Pat. Nos. 2,581,414 and 2,855,156.
The present inventors have studied mixing and kneading methods to improve the dispersibility in order to be able to obtain a magnetic tape having higher S/N ratio.
As described in the foregoing, a two-roll mill, a three-roll mill, an open kneader, a pressure kneader, and a continuous kneader are known for mixing and kneading the coating composition for a magnetic tape. However, using any one of these devices, the degree of mixing and kneading depends upon the quality of the ferromagnetic particles, and the relative quantities of binder and solvent, and it is extremely difficult to prepare a coating composition having the desired dispersibility. The difficulty increases as the particle size of the ferromagnetic particles becomes smaller. Thus it has been difficult to obtain a magnetic tape having better dispersibility and a higher S/N ratio when ferromagnetic particles having smaller particle size have been used and dispersed by a conventional method.